Calibration Report for the TPB Travel Forecasting Model, Version ...

Calibration Report for the TPB Travel Forecasting Model, Version 2.3
1.3.4 Subdivided non‐home‐based purpose
Whereas the Version 2.2 model uses a single Non‐Home‐Based trip purpose, the Version 2.3 model
disaggregates NHB travel among two sub‐purposes: Non‐Home‐Base Work (NHW) and Non‐Home Base
Other (NHO). This change was also in line with consultant recommendations. 10 TPB staff felt that the
observed differences between these travel markets in terms of trip rates, trip lengths, modal
preferences, etc. were substantial enough to justify establishing an additional purpose. TPB staff
considered making other splits of trip purposes (e.g., separating out home‐based school trips from
home‐based other trips), but ultimately decided to defer making any further splits of trip purposes at
the present time.
1.3.5 Refined non‐motorized travel
The Version 2.3 model includes the development of non‐motorized trips 11 for all (work and non‐work)
purposes. The Version 2.2 model develops non‐motorized travel for the HBW purpose only. Staff felt
that the more detailed TAZ system would facilitate efforts to better reflect this particular travel market.
However, the non‐motorized travel will be developed at the trip generation stage only.
1.4 Overview of the Version 2.3 travel model
A graphic showing the essential parts of the Version 2.3 modeling process is shown on Figure 2. Despite
the general name for travel models (“four step”), the TPB travel model could more accurately be called a
“six step” model. These six steps are described below.
The demographic models are used to disaggregate the total number of zonal households across 64
cross‐classes: 4 household income groups 12 by 4 household size groups (1, 2, 3, 4+ persons) by 4 vehicle
availability groups (0, 1, 2, and 3+ vehicles available). The allocation of households to each cross‐class is
made at the traffic analysis zone (TAZ) level. The figure indicates that peak‐hour transit accessibility
measures are used as part of the demographic (vehicle availability) submodel step.
The trip generation models are next applied to compute daily person trip productions and attractions by
purpose. Five modeled purposes are modeled: Home‐Based Work (HBW), Home‐Based Shop (HBS),
Home‐Based Other (HBO), Non‐Home‐Based Work (NHW), and Non‐Home‐Based Other (NHO). A
commercial vehicle purpose (consisting of both autos and light duty trucks), and two truck types,
Medium and Heavy, are also modeled. Medium trucks are those with two axles and 6 tires. Heavy
trucks represent all combination vehicles.
10 Cambridge Systematics, Inc., Fiscal Year 2010 Task Reports, Final Report (Washington, D.C.: National Capital
Region Transportation Planning Board, November 16, 2010), 2–3, 2–12.
11 Actually, “trip ends,” since they are an output from trip generation.
12 The income levels used approximate household income quartiles, based on the 2007 ACS.
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